In recent years, the separation of components or mixtures either in liquid or gaseous form has attracted a great deal of interest. For example, reverse osmosis may be used in the purification of water, especially saline water. When utilizing reverse osmosis in the purification of saline water, a pressure in excess of the osmotic pressure of the saline water feed solution is applied to the solution which is separated from purification water by a semipermeable membrane. Pure water thereby diffuses through the membrane while the sodium chloride molecules or other impurities which may be present in the water are retained by the membrane. Likewise, brackish water which contains impurities other than or in addition to sodium chloride may also be subjected to a reverse osmosis process utilizing similar semipermeable membranes. The specific types of semipermeable membranes which are employed may be fabricated from cellulose acetate, polyamides, polyimides, polyphenyl esters, polysulfonamides, polybenzoimidazole, polyarylein oxides as well as other polymeric organic material.
It is taught in U.S. Pat. No. 4,243,701 to Riley et al. that certain membranes may also be utilized for the separation of various gases. The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of the gas across the surface of the membrane. Inasmuch as the feed stream is at an elevated pressure relative to the effluent stream, a more permeable component of the mixture will pass through the membrane at a more rapid rate than will a less permeable component. Therefore, the permeate stream which passes through the membrane is enriched in the more permeable component while, conversely, the residue stream is enriched in the less permeable component of the feed. U.S. Pat. No. 4,230,463 discloses a multicomponent membrane which is useful for separating gases. The membrane comprises a polymer coating on a porous separation membrane, in which the latter membrane may itself also be a polymer such as a polysulfone. It is to be noted from this patent that the polysulfone which was employed is not unduly porous and has a narrow range of ratios of total surface area to total pore cross-sectional area. In particular, the patent discloses the use of membranes having ratios of total surface area to total pore cross-sectional area of about 1000:1. This type of membrane is considered a tight membrane having relatively small diameter pores which is in contrast to the polymer support of the present invention which possesses a total surface area to total pore cross-sectional area in a range of from about 5:1 to about 800:1. While the type of membrane set forth in the patent may be conducive to high separation factors, the rate of passage of fluid through the membrane which is the flux is restricted.
As will hereinafter be shown in greater detail, it has now been discovered that membranes which are relatively loose, i.e., membranes having relatively large pores may, after treatment thereof, be utilized in a gas separation process in an economical manner.